Manufacture of pulp



Dec. 3, 1935. c. K. Tl-:xToR

MANUFACTURE OF PULP Filed sept. 1e, 192e msm,

Patentled ``3, V1935 I A Clinton I. Textor, .Cloqnct, lIinn., assigner to a Northwest Paper Company, Gloquct, Minn., 'a'

corporation of lMinnesota yApplication september 1s, 192s, sonni No. 135,929

y Claims. 'I'his invention relates to improvements in wood digestion with normal suliltes, the pulp produced,

bered woods.

the recovery of the chemicals in the resulting liquor,Y and the preparation of the cooking liquor. Although these improvements apply to woods in general, the presentapplication is directed more particularlyto the preparation fof pulp from con-iferous woods, such'as pines, lilrs, spruces, hemlocks, redwoods, Sequoias; Douglas firs and latches, which are loosely'characterized as long- In aco-pending application Serial No. 135,930 filed oneven date herewith, I have disclosed and y claimed`\ these improvements as directed more particularly to woods, such -as poplars, gumwoods, maples, oaks,` elnis,l basswood, and all such woods as aregenerallyincluded under the term hardwoods", and .sometimes loosely designated as short-bered-woods.

Among the objects of my placing with `an alkali carbonate, such as carbonate -or bicarbonate, of the caustic soda heretofore deemed necessary when cooking wood with sodium sulte liquor, the regulation of-the 'carbonate in the cooking liquor to minimize the corrosion of the equipment without "adversely a'ecting the economy or the efllciency of the process, 7the improved method of making,

. pulp from highly resinous woods, and the treatment of the so-called black liqior resulting from the cooking operation so that it may be highly ,concentrated in7 eilicient tubular evaporators without plugging these evaporators'by separation of solids.

other objects andlndvnntnges of my inven-v tionwillbeapparenttothoseskilledintheart from the following speclncation. The accompanying' ligure shows, more or less diagramtration is glven'only for the purpose of facilitat-l matically, one arrangement of apparatus capable of carrying out the procs-of this invention; but it is .to be understood that this illusm""1aao, cmu made known un: uuntyfor vsodium suliite'as a wood digestant.. Since that number o modiilcations have' appeared in uio literature .and in patents of the United Canada and Great Britain.

"This art shows that it is'considered n,

when using an essentially sodium t cooking liquor, comprising, principally, the normal invention are the\re (Cl. 92--1l) soda (NaOH) to dissolve the silicates and aluminates found 'in cellulose bearing materials, such as wood. Schachts German Patents Nos. 122,171 and 131,118 ,specic'ally require the presence of small amounts of caustic soda. Other investigators have deemed it necessary' to have still larger amounts of caustic soda present in the normal sodium sulilte cooking liquor, especially` when` ,highly resinous woods are used.v I have found that it is not necessary to have caustic soda (NaOH) present in such a sulilte liquor to pro- `duce a satisfactory pulp, and that there are certain advantages in operation and certain improvementsin the pulp resulting directly from replacing ythe stronger caustic soda of the prior art with a milder alkali carbonate, such4 as sodium bicarbonate, or sodium carbonate.

By my method of cooking, which is described in detail hereinafter, I obtain a high yield'of well cooked pulp, in some c as high as 50%,. (for long-bered wood) to 60% (for short-fibered wood). of the weight of the wocd employedfbothbeing calculated on oven dried material.- The,

product thus produced is of specially good strength, has an unusually white color for. unbleached pulp, and 'possesses those characteristics essential for the manufacture oi strong durable paper of the highest grade. The 'pulp is 'readily bleached to a good white color, the -bleached pulp retaining practically all of the desirable properties of the unbleached pulp.

Although several -modiilcations are possible without,departing from the spirit o! -my invention, I prefer to carry out the cooking process -as described in detail hereinafter'.V In considerdisclosed, itis toV be 'understood that the temperature and the proportions of material to wood may be varied according to the type of wood, the

"moisture content of the wood, the kind o! pulp (Nason, to have present caustic,-

desirod, and other -rnotors horoinnzzeruesnribed, as carried out with Jack pine.

fThe digester I isllliodiiuthcl desiredbiht with jack pine chippcdto a convenient sise, and the cooking liquor'is added in volumes', similar tothoseusedforcooking-woodbythesodmor the sulfate process,'as is well known to those versed 1n the nrt'. I und it desirable to nda from one-quarter to three-'quarters cubic foot of cook-V ing Aliquor per cubic foot of chips as they lie in ingv the exemplary ngures and details hereinafter the digest'er, depending'upon vthe moisture content or the wood, the method 'f circulating the liquid, the -method of heating, and other factors. The preferred liquor isoIsuch-a concenblack liquor, more particularly described herein' after, or ,black liquor containing washingsdnay be used in place of water without any detrimental A results.

The detailed method of preparing the cooking liquor either as a fresh solution or as a regener- .ated solution is disclosed more particularly in the ensuing description. Such cooking liquor has beenr found to have alkalinity equivalent to a hydrogen. ion concentration expressed in the terms of pH of from about 7.5 to 9.5, and at no time during the cooking processdoes this liquor become acid.- It is understood that neutralitybetweenacidity and alkalinity is to be referred to water which has a pH equal to approximately '7.0, and, further, that Aall pH values herein given are measured at laboratory temperatures. 4I have found that a measurable alkalinity is required to minimize corrosion to such an extent 'that an unlined steel or iron digester may be used without excessive corrosion. It is not necessaryto use more than the above mentioned amount of alkali carbonate to safeguard against" excessive corrosion. In using greater quantities of carbonate no additional advantages have been "observed. It is important that the carbonate em-,l ployed be present in addition to the amount of sodium suliite required and not in substitution therefor. ered necessary for the process is deemed otherwise `important in at least one particular circumstance. I have found that the preferred liquor on the` quantities stated must contain sufiicient sodium suliite (NazSOa) to prevent the production of black or dark brown pulp by any sodium thiosulfate (NazSaOs) which may be present as an impurity, as hereinafter explained. It appears that if the normal sulte becomes exhaustedin the process of the digestion, the thiosulfate then acts predominantly upon the pulp disadvantageously, producing a dark colored product.

' The cooking process is preferably carried outv in an unlined steel digester by a procedure similar to that of the soda or a 'modified so'da process. By a soda process, I refer to one which employs caustic alkali (NaOH) `as the pulping agent, while by a modified-soda process, I refer to a soda process in which the liquor consists chiefly of caustic alkali, but to which sodium sulfide the digester.

(NazS) or sodium sulfite (NazSOi)l or other materials have been added. A temperature Vof' 175 to C. is -preferred and the corresponding saturated steam pressure is maintained within These temperatures are not to be construed as limits, because a temperature of 188 has been successfully employed. -Inv using the above quantities of material, it is to be understoodl that the digester is heated by introducing steam directly into it. Heating :nay also be done indirectly by a.v suitable adjustment in the volume and concentrationof the liquor-employed. The time of cooking generally varies from six to twelve hours, depending upon the grade of pulp desired 'and the conditions of cooking. Forexample, when' a wrappingpaper pulp` is desired,

The quantity of sodium sulie consid-l from the liquor resulting from the digestion, this l0 liquor hereinafter being referred to as black liquor. The separation is herein accomplished by means of a continuous filter, indicated atr 4, from which the pulp is discharged in one direction and the black liquor in another. 'Ihe pulp 15 is then subjected to a thorough washing and lscreening operation, as indicated generally at 5,

to remove undigested pieces of wood and to free it from traces of the discoloring black liquor.'

Thorough washing is essential for the production 20 of a pulp with a good natural colorfwhich is easybleaching. 'I'his washed pulp may, if desired, be

. used for paper without further treatment, as

indicated at 6. Such a pulp.. as made by this process, has been very slightly acted upon bythe 25 cooking liquor; it has a. very light tan color comparable to easy-bleaching sulfite pulp, but possesses a strength equivalent to that of a good kraft pulp. The pulp is practically resin-free, and

is capable of being readily semi-bleached by 30 treatment with about 16% to 20% (based on the air-dried 'weight of pulp) bleaching powder containing 35% available chlorine, and it may be fully bleached with about `30% to 40% of such bleaching powder to form a brilliant, lustrous 35 color equivalent to, and usually better than, stock prepared by an a'cid sulte process. By an acid suliite process, I refer to one in which the cooking solution contains about four to six percent of sulfur dioxide, of which 1% to 1%% is oombined with a base, such as sodium, Acalcium or magnesium, the rest being present at free"'sul furous acid. In the semi-bleaching operation,

above referred to, the pulp does not losef. its

strength. .-The results of strength tests made on 45 semi-bleached pulp show that the stock is even slightly stronger than the unbleached pulp. The

vfully bleached pulp, above referred to, has a color equivalent to, and a strength 25% to 35% greater,

l than, the best bleached suliite pulp. Corsequent- 50 ly, the pulp of the present processlends itself to a'large number of uses inthe paper mill, as it possesses a desirable unbleached color and has the bleachability of sulilte pulp, and the high strength of kraft pulp. bleachingis represented at l, and the sulsequent washing at 8, such pulpbeing used for paper of high grade, as shown at9. l

Another feature of the process. is the cyclic operation as regards the base metal of the'diges- 60 tion liquor. Since sodium is by flar the cheapest and the preferred base, the process will be explained in detail with reference to the said liquor.

In general, the preferred steps of regeneration 65 .comprise concentration of the black liquor resulting from the digestion, incineration to vburn outithe organic matter, leaching of the ash from the incineration, giving substantially a sodium carbonate (NazCOs) solution, and sulilting of the 70 leach liquor with sulfur dioxide (S01) to form y sodium suliite (NazSOa) and to'leavea predetermined -amount of unsuhlted soda, preferably as .sodium bicarbonate (NaHCOa) In the concentration of the liquor, I 75 Inthe diagram, the55 have used a quadruple effect high speed tubular evaporator in which the upper part of the body of liquid being evaporated is contained in the lower ends of a series of tubes, as represented in the drawing at I0. Considerable trouble has been experienced in so evaporating this liquor because it deposits solid during the concentration. The solids deposited are, primarily, organic compounds which adhere to the tube walls and gather on the bottom of the evaporator, making it necessary to clean the latter several times a day. Such a condition prevents continuity in concentrating and is generally very undesirable, whatever type of evaporator is used.

I have found that the addition of a caustic alkali, such as sodium hydroxide (NaOH), to the black liquor prevents the deposition of such solids, and permits continuous evaporation to be carried out in the system described to a concentration` of 36 to 40 B. (45% to 55% solids) measured at 60 F. Thel evaporator can thus be operated continuously for several weeks as compared to a few hours. The addition of alkali may be made just prior to the entry of the black liquor into the evaporator, at the point il in the drawing.`

The addition of caustic alkali leads to numerous unexpected advantages. It is customary n most cyclic processes to supply at some point fresh material to replace what is incidentally lost throughout the cycle. Since sodium is the constituent which is re-cycled, the addition of sodium hydroxide for the purpose above described also provides the necessary amount of replenishing material. The substance added may `be pure caustic soda, solid or in solution, but, inasmuch as 'the black liquor isheavy with impurities and is later subjected to purifying operations, a very impure, and, hence, very cheap caustic soda may be employed. Consequently, it is a general practice to employ a waste liquor from some other process which will provide the required caustic soda, such, for example, as the black liquor from some soda process of pulp formation.v

The effect of the caustic soda is to increase the alkalinityV of the black liquor to be concentrated. Tests on several black liquors show an increase in alkalinity from pH 8.5 to about pH 9.2 upon addition of caustic soda through the medium of black liquor from a caustic process and a further increaseto pH 9.6 when additional caustic alkali (NaOH) is added to ythe mixed black liquors. The amount of caustic to be added is immaterial so long as a sufficient amount is added to avoid the troublesome conditions above described. I'Ihe result desired is the guide to the amount required and this varies with the type of wood for which the liquor has been used, the time of cooking and other specific conditions.

The syrupy mass or concentrate from the evaporator may be dried or run directly to an incinerator, var-ious types of which are wellE knwn to the art. Since the organic content of the concentrate is sufficient to provide, upon combustion, the necessary heat to operate the mill, it is quite important that it be in the so-called soluble condition effected by the caustic addition. The.

alkaline concentrate, acting as a vehicle for the organic matter, permits it to be pumped' and conitsremoval, the problem of disposal, and the uncertainty of the operations.

One method of recovering sodium carbonate from the black liquorconcentrate is partly to incinerate the ash, not to completion, however, and

to transfer the partly incinerated ash to a smelter which completely removes the carbonaceous material and discharges a fused sodium carbonate containing some impurities. This smelt is dissolved to form essentially a sodium carbonate solution.

The preferred method of treating the black liquor concentrate is to pass it through an incinerator I 2 without approaching the fusion state. This results in an ash which is primarily sodium carbonate with some free carbon, depending in amount upon the extent to which it has been burned. Furthermore, it may contain sodium sulde (NazS) formed by the reduction of certain of the sulfur compounds, and small amounts of other substances. The incinerator ash is leached, as indicated at i3, with water or with some suitable solution which is to be enriched in sodium carbonate (NazCOa). The leach liquor is then filtered through a suitable lter, as indicated at I4.

I have found that sawdust i5 is a most satisfactory filtering medium. It has the advantageous prop-.

erty of removing suspended solids, chiefly carbon, without clogging. It is more eillcient than sand in this respect, being lighter, more porous, and has a higher specific surface.' I have used the sawdust in a bed to a depth'of about three feet, the sawdust bed being coniined or weighted down, as at I6, to, guard against floating when immersed. The tendency of sawdust to float, being greater than that of sand, causes the bed to rise or expand rather than to pack when used in a tank with a drainer bottom il.

In practice, I have found it economical and otherwise beneficial to prepare the iiltered leach liquor from the ash of the concentrated combined liquors obtained both from the digestion step of the present invention and from the old and well known caustic soda digestion processes. In Table I, following, there are shown analyses of two samples, designated A and B, each being a composite of forty samples of such leach liquor.

tion, all sulfur would be roasted out leaving a pure sodium carbonate ash which would yield an ideal leach liquor containing only sodium carbonate (NazCOa) Practically, such an ideal condition is not attainable, and, furthermora'it is not necessary. However, when desired the effect of these impurities may be minimized by a special treatment of the leach liquor when it is, for

any reason, desired to approach the ideal con-I dition. The sulting of the leach liquor with sulfur dioxide (SO2) initiates a series of reactions forming sodium thiosulfate (NazSzOs) from sodium sulfide (NazS) This formation of sodium thiosulfate may be prevented by treating the leach liquor with carbon dioxide (CO2) taken,

for example, from ilue gases forming hydrogen sulilde (HzS) and sodium bicarbonate. The normally gaseous but water-soluble hydrogen sultlde is retained by the solution being treated. This condition effects a counter-reaction requiring the use of a considerable excess of carbon dioxide to remove the hydrogen sulfide (HzS) from solution and thereby to permit continued conversion of the sodium sulfide into the bicarbonate.

As there are large amounts of carbon dioxide available from flue gases, the excess required is no practical disadvantage to this operation from a chemical standpoint. From the standpoint of mechanical operation, however, it is possible to Aview this carbon dioxide treatment as an extra operating step in the regeneration. Such is not the case, for I havefound it possible to combine this step with the sulfiting step.

The leach liquor containing thesulfide is run against a counter-current of gas containing sulfur dioxide and carbon dioxide, the latter being preferably in excess, so that substantially all the sulfur dioxide is absorbed whereby to permit carbon dioxide toilow on. Thus, the excess of carbon dioxide meets the incoming leach liquor, which at this stage may be rapidly 'fe-circulated in the path of the carbon dioxide. fihe carbonio acid gas converts both the sodiumsulde and the sodium carbonate to bicarbonate, and removes the resulting hydrogen sulde. T hel sodium bicarbonate then absorbs the sulfur dioxide forming the normal sulte, the extent of this reaction being controlled to form the 4desired composition of the emergent solution.

In the drawing, this treatment is represented as optional by dotted lines. An absorption tower I8 receives the leach liquor at the top, and a stream of mixed gases enters the bottom of the tower. These gases are preferably taken from the incinerator I2, thus to effect a substantial recovery of the sulfur employed in the process, and an advantageous use of the carbon dioxide from the combustion of the black liquor concentrate.

The better practice, however, is to so effect the incineration of the black 'liquor that the impurities occurring in the leach liquor may be neglected.v In such a case, the leach liquor is sulflted directly by passing sulfur dioxide gas into the absorption tower I8, as represented at I9, until the liquor exhibits a predetermined composition of sodium sulte (NazSOa) and of unsulted soda. A part of the original sodium carbonate will not be sulted; but it will be acidifled to sodium bicarbonate by action of the sulfurous acid (HzSOa), formed upon the solution of the sulfur dioxide gas in the water. When the bicarbonate content is in the region of 0.50 to 0.75 pounds per cubic foot of solution, the

sulting is deemed complete for the preparation.

`rthe following reactions are given:

In view of the above and of the preceding description, it should be understood that all the reactions are allowed to go to completion, except reaction I. The supply of S02 forming H180: the cooking, which Table II Pounds per cubic loot Sodium bicarbonate 79 92 Sodium sulte l2. 40 l?. 70 Sodium thiosuliate. 2. 08 98 Sodium sultate 1. 07 l. 00

Such a cooking liquor containing impurities, as

above displayed, is quite suitable for the cooking of wood by my method. It has not been observed that impurities to the amount above stated have any` appreciable effect upon the quality of the pulp produced over that which is produced y in the absence of such impurities. made to the presence of these contaminating materials, and it is to be understood that they are merely incidental to a practical operation of the process hereinafter more particularly delned in the appended claims.

It should be explained at this point that heating a bicarbonate solution will cause reaction 3 to proceed in the reverse direction forming carbonic acid which breaks down into water and No claim is v gaseous water-soluble carbon dioxide (CO2).

Since sodium bicarbonate is used in the digestion and is under heat, it is evident that it breaks down forming carbon dioxide and sodium carbonate (NazCOs). The carbon dioxide is released from the digester from time to time along l with other gases `formed in the process. It is difficult then to ascertain the ratio of normal carbonate to bicarbonate during the ,cooking process. The escape of the acid constituent (CO2) during digestion thus permits an in-` crease in the alkalinity due to sodium carbonate (Nar-CO3). Such an increaseI has been observed as will hereinafter be explained.

The cooking liquor containing essentially sodium4 sulte and sodium bicarbonate, and incidentally a number of impurities as exempliiied by Table II, initially has a measured alkalinity of a pH of about 7.5 to '9.5. After it has been employed in wood digestion and is recovered as black liquor, itsv pH value is about 8.5 to 10.5. At no time during the cooking operation does the pH value fall below substantially '1, which is the dividing line between alkalinity and acidity. This is an important feature of the invention, since liquors which are below pH 'l are considered as being acid` and to have .a corrosive action on steel digesters. I have found by experience that when the alkalinity is maintained above pH '7.5 there is a minimum of corrosion. It has been observed that under these conditions a black adherent lm forms on the steel digester, and this lm apparently aids in preventing the cooking liquor from attacking the underlying steel. It has further been observed that if the liquor has an initial pH less than 7.5, the alpermits corrosion in spite of the black nlm.' Long experience has shown that this lowering of the alkalinity during the i'lrst part o! the cooking process is not permanent, and that the alkalinity subsequently rises again duringv the continuance of the cooking operation. The rise is due in all probability to the escape of the acid constituent CO2, acetic acid and other gases, probably acidic in character, as heretofore explained.` The decrease or drop during the initial stagesfmay be explained by a variety of hypotheses, which, however, will be omitted. "It is suiiicient to state that the alkalinity decreases and corrosion occurs if the initial alkalinity is too low. Consequently, sui'- iicient alkali carbonate is initially present to give a pH of 7.5 or greater. A larger quantity'may be used, but apparently no additional protection is obtained'.

The foregoing description of the cooking liquor from leach liquor. It is, oi course, to be understood that it may be prepared by sulilting a sodium carbonate solution prepared from the commarcial salt (NazCOs). For example, a solution which cannot easily be avoided. However, whenthe'.salts are mixed` withoutv a sulilting operation, the cooking liquor may contain normal sodium carbonate and sodium suliite, the sodium y bicarbonate not being essential when the knormal 'carbonateis present. In the practical operation of the process it is most desirable to regenerate theliquor, butthe process, chemically, is not dependent upon this typ'e of liquor. A fresh liquor may be used when recovery steps, not, only to receive waste mater'ials'from other processes, but also toprovide material for userin other processes. This permits conjoint operation of several distinct digestween them, and permits, further, the use of the v same apparatus -for the conjointly operated processes. Not only may the several processes v be `simultaneously operatechbut one `may be G Sreadily followed by another because ofithis advantageous relation. Such a conditionreduces the amount fof equipment requiredA and gives a Well.v equipped plant a wide nexibility in the use` of apparatus and processes. For @ampiahas been directed, generally, to that prepared tion processes withfa minglingof materials bebase as used for the process of this invention.

the' leach liquor is deemed too high in sulfide for the better requirements oi the present invention, it mayA he readily diverted into the sulfate-process, witlrout economic loss, thus to eliminate a special treatment toy reduce the 5 sodium sulte content. Furthermore, ,the alkali carbonate instead of being `incompletely suliited may be converted to the acid sulilte and employed in an acid sulnte process, having the same In the fore part of this description, reference was made to certain disadvantages of the soda Y or modified soda processes which are overcome by the present inventionr especially those using caustic soda and-sodium sulilte. One of these 15v isl 'the danger toA humans of caustic-solutions, which are very corrosive on the skin as compared to carbonatos. Another is the dimculty of preparing the sulnte-caustic soda solution.

0n regeneration of cookingliquor for such 20 caustic soda processes from blackliquorthe car-'- bonate is formed. This carbonate requires in addition tothe suliiting operation acausticizing operation. I eliminate this causticizing step by retaining the carbonate unsuliited. The causticizing step is usually necessaryeven though in the preparation of the cooking vliquor a causticsoda is used initially insteadofoarbonata because commercial caustic usually contains or readily forms a carbonate as impurity. Itis customary to causticize this impurity. These causticizing operations when conducted in thev presence of alkali sulte present technicaldifdculties,

V the description oi which will be omitted as forming no part of this invention. When an attempt 3'5 is made otherwise to elixriinate the causticizing operation, by over-sulntlng a soda solution and then adding caustic, a new condition is encountered. The sulilting of, the soda liberate/s carbonio acid gas4 (C01) which remains dissolved in the 40 solution. This Agas can .only be removed with great operating diillculty andgitslremoval is necessary for economy in the addition of the caustic soda. Thus, the present invention, besides prviding a new and advantageous pulping` process, alsoeliminates ucausticizing operations and the CO2 removal ,required by other processes, and, turthermore, ssimpliiiesV thepreparation oi.' the cookjing liquor bythe regeneration.method.

'I'heflight color oi' the product is one of the 50 prominent features recommending the use of this process. The black liquor contains s'.l compound -which isa .chemical-indicator like litmus, chang.- ingcolor. at some dennite pH value. The. compound referred to darkens at a `pI-l value higher 55 than that imparted yby my solution` as presentat. the time4 such indicator is formed. If the. solution is sumciently alkaline to darken the' indicator,

Ckalinity .in the presence of black liquor.

- and measuring thepercentfo! light by 75 the latter colors thepulp and the color cannot be -readily washed out.y Ii?y thefpuip is washed prior to a darkening of the indicator, the latter :ls-,quite readily removed. Hence, to produce a lightiulp itis important to prevent a high -al- M The elimination of strong caustic sodlirom the line processes and its replacement by a milder carbonate contribute largely to the causes o! light color of the pulp. As a measure of the pulp colo/"r, the following examples are. given in Table in which E is' a'sample of-unbleachedtama rack and-E is 'a sample of semi-bleached tama-'- rack. The method used to\detemi.1 ne 'the 'color consists 'informing a sheet of the pulpI sample wave lengths in ngstrom umts.

the sheet at different colors, expressed as their Table IH Percent reilection 460 (Blue and indigo) 480 The jack pine pulp is leven lighter than the tamarack, above exempliiled.

It is to be understood that the process is not,

limited to use with one kind vof wood, such as the jack pine herein described. Not only may diiferent woods of the same class, but woods of different classes may be cooked as mixtures, such as mixed poplar and pine. For example, I have successfully cooked birch and tamarack, the former a short-fibered hardwood, and the latter alongbered coniier. The cooking operation is extended to the period required for the slowest cooking wood without apparent damage to the pulp of the more rapidly cooking wood.

The pulp of such a mixture gives rise to certain operating advantages, especially when it is processed with a continuous filter discharging a cake or heavy sheet ofpulp from a suction` roll. The long fibers alone produce a sheet which readily breaks, whereas the presence of short ilbers tends to strengthen the sheet by filling in the smaller spaces. f

'Another object in mixing woods for common digestion is to improve the color oi the darker and stronger type oi' pulp by the presence of a lighter and weakertype of pulp. For example, when ten to twenty percent of birch is used with eighty to ninety percent of tamarack, theA resultant pulp has been Afound to be as strong, within comercial limits, as the tamarack pulp alone, and to have a color considerably lighter than the tamarack pulp alone. It is not at all customary to mix short-bered hardwood pulp into longand pine,Y woods of the same class, have aA strength disproportionately nearer that oi.' the stronger pulp when both are made by my proc ess, and that they have a strength almost proportionate when both are made bythe sulfate or kraft process. 4

Other advantages of cooking mixed woods oi' different classes will be obvious to those skilled in theA art.v v 7 In the foregoing, I have described the process with reference to a sodium base, but it is understood that the chemically equivalent bases or the mixed bases are contemplated as will appear .from the character of tle appended claims.

What I claim as new is:

1. The method of making substantially lignin 1 free cellulosepulp from wood, which comprises vdigesting the wood with a solution containing alkali salts of sulphurous and carbonio acids as the two essential chemical ingredients, sumcient. sulphite being used in excess of the amoimt required to release the cellulose to prevent 5 staining. v A

2. The method of making substantially ligninfree cellulose pulp from wood which comprises digesting the wood with a solution containing sodium salts of sulphurous and carbonio acids as 1othe two essential chemical ingredients, sutlicient sulphite being used in excess of the amount required to release the cellulose to prevent stain- 3. The method oi' making substantially lignin- 15 u free cellulose pulp from wood chips which comprises digesting the wood until the desired cellulose is released with an alkaline sodium sulflte cooking liquor of an alkalinity not less than pH '7.5, which contains 40 to 45 pounds of sodium 20 sulfite and 1.5 to 3.0 pounds of sodium bicarbonate per pounds of oven dried wood, the concentration of the liquor being such that 1A to cubic foot of liquor is used per cubic foot of woodv chips in the digester, at a temperatm'e of 25 from tof'185 C.,'and at the corresponding saturatedsteam pressure for a period of from six to twelve hours, o'f which the iirst one and one-half to three hours is used in attaining said temperature and pressure.

4. The method of making substantially lignini'ree cellulose pulp from wood chips which comprises digesting the wood until the desired cellulose is-released with an alkaline sodium suliite cooking liquor of an alkalinity not les than pH 35 7.5, which solution contains 40 to 45 pounds of sodium sulilte and a suilicient quantity of a.v sodium salt of carbonio acid to produce the ref quired alkalinity per 100 pounds of oven dried chips, concentration oi! the liquor being such 40" sodium sulphite, the sodium salto! a weaklyo ionized -polybasic acid and lanexces oi' carbon dioxide under temperatures up to 190 C.

6. The process of pulping wood which consists of treating it with `solutions Ycontaining sodium sulphite, sodium bicarbonate and an excess o! 55 l carbon dioxide under temperatures up to 190 C.

'7. 'I'he process of pulping wood which consists of treating it with a solution sodium sulte, the sodium salt Aoi! a weaklyV ionized polybasic acid;- andan nl carbon dioxide under temperatures -up to 190,Y C., the

sodium `su1te being presentoin sumcient qusntity to dissolve the lignin of the wood, release cellulose-and prevent staining. 5

8. The process of pulping wood which .of treating it with a solution containing sodium sulfite, sodium bicarbonate, andV anf exce ot carbon dioxide vunder temperatures up to 190* C.,

the scdium sainte being present :new

basic seid, and an excess of carbon dioxide sodium sulte, sodium bicarbonate, and an eir-` under temperatures up to 190 C., the sodium cess: of carbon dioxide under temperatures up to sulte being present in excess of the quantity re- 190 C., the sodium sulte being present inl quired to dissolve the lignin of the wood, release excess of the quantity required to dissolve the 5 cellulose and prevent staining. lignin or the wood, release cellulose, and prevent 5 10.-'Ihe process of pulpinz wood which constaining. sista o! trestingit with a solution containing CLINTON K. TEXTOR. 

